4.1 general superseded - government of new jersey...4.3 horizontal alignment 4.3.1 general in the...

BDC07MR-01

NJDOT Design Manual – Roadway 4-1

Section 4 – Basic Geometric Deign Elements

Section 4

Basic Geometric Design Elements

4.1 General

Geometric highway design pertains to the visible features of the highway. It may be

considered as the tailoring of the highway to the terrain, to the controls of the land usage, and to the type of traffic anticipated.

Design parameters covering highway types, design vehicles, and traffic data are included in Section 2, “General Design Criteria.”

This section covers design criteria and guidelines on the geometric design elements that

must be considered in the location and the design of the various types of highways. Included are criteria and guidelines on sight distances, horizontal and vertical alignment,

and other features common to the several types of roadways and highways.

In applying these criteria and guidelines, it is important to follow the basic principle that

consistency in design standards is of major importance on any section of road. The highway should offer no surprises to the driver, bicyclist or pedestrian in terms of geometrics. Problem locations are generally at the point where minimum design standards

are introduced on a section of highway where otherwise higher standards should have been applied. The ideal highway design is one with uniformly high standards applied

consistently along a section of highway, particularly on major highways designed to serve large volumes of traffic at high operating speeds.

4.2 Sight Distances

4.2.1 General

Sight distance is the continuous length of highway ahead visible to the driver. In design,

two sight distances are considered: passing sight distance and stopping sight distance. Stopping sight distance is the minimum sight distance to be provided at all points on multi-lane highways and on two-lane roads when passing sight distance is not

economically obtainable.

Stopping sight distance also is to be provided for all elements of interchanges and

intersections at grade, including driveways.

Table 4-1 shows the standards for passing and stopping sight distance related to design speed.

4.2.2 Passing Sight Distance

Passing sight distance is the minimum sight distance that must be available to enable the

driver of one vehicle to pass another vehicle, safely and comfortably, without interfering with the speed of an oncoming vehicle traveling at the design speed, should it come into view after the overtaking maneuver is started. The sight distance available for passing at

any place is the longest distance at which a driver whose eyes are 3.5 feet above the pavement surface can see the top of an object 3.5 feet high on the road.

Passing sight distance is considered only on two-lane roads. At critical locations, a stretch of four-lane construction with stopping sight distance is sometimes more economical than two lanes with passing sight distance.

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Table 4-1

Sight Distances for Design

Design

Speed Mph

Sight Distance in feet

Stopping

Minimum

Passing* Minimum

25

30 35

40

45 50

55

60 65

70

155

200 250

305

360 425

495

570 645

730

900

1090 1280

1470

1625 1835

1985

2135 2285

2480

* Not applicable to multi-lane highways.

4.2.3 Stopping Sight Distance

The minimum stopping sight distance is the distance required by the driver of a vehicle,

traveling at a given speed, to bring his vehicle to a stop after an object on the road becomes visible. Stopping sight distance is measured from the driver's eyes, which is 3.5

feet above the pavement surface, to an object 2 feet high on the road.

The stopping sight distances shown in Table 4-1 should be increased when sustained

downgrades are steeper than 3 percent. Increases in the stopping sight distances on downgrades are indicated in AASHTO, “A Policy on Geometric Design of Highways and Streets.”

4.2.4 Stopping Sight Distance on Vertical Curves

See Section 4.4.4 “Standards for Grade” for discussion on vertical curves.

4.2.5 Stopping Sight Distance on Horizontal Curves

Where an object off the pavement such as a longitudinal barrier, bridge pier, bridge rail, building, cut slope, or natural growth restricts sight distance, the minimum radius of

curvature is determined by the stopping sight distance.

Stopping sight distance for passenger vehicles on horizontal curves is obtained from

Figure 4-A. For sight distance calculations, the driver's eyes are 3.5 feet above the center of the inside lane (inside with respect to curve) and the object is 2 feet high. The line of sight is assumed to intercept the view obstruction at the midpoint of the sight line and

2.75 feet above the center of the inside lane. Of course, the midpoint elevation will be higher or lower than 2.75 feet, if it is located on a sag or crest vertical curve respectively.

The horizontal sightline offset (HSO) is measured from the center of the inside lane to the obstruction.

The general problem is to determine the clear distance from the centerline of inside lane

to a median barrier, retaining wall, bridge pier, abutment, cut slope, or other obstruction for a given design speed. Using radius of curvature and sight distance for the design

speed, Figure 4-A illustrates the HSO, which is the clear distance from centerline of inside lane to the obstruction. When the design speed and the clear distance to a fixed obstruction are known, this figure also gives the required minimum radius which satisfies

these conditions.

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When the required stopping sight distance would not be available because of an obstruction such as a railing or a longitudinal barrier, the following alternatives shall be

considered: increase the offset to the obstruction, increase the horizontal radius, or do a combination of both. However, any alternative selected should not require the width of the

shoulder on the inside of the curve to exceed 12 feet because the potential exists that motorists will use the shoulder in excess of that width as a passing or travel lane. This is especially pertinent where bicyclists can be expected to operate.

When determining the required HSO distance on ramps, the location of the driver's eye is assumed to be positioned 6 feet from the inside edge of pavement on horizontal curves.

The designer is cautioned in using the values from Figure 4-A since the stopping sight distances and HSO are based upon passenger vehicles. The average driver's eye height in large trucks is approximately 120 percent higher than a driver's eye height in a passenger

vehicle. However, the required minimum stopping sight distance can be as much as 50 percent greater than the distance required for passenger vehicles. On routes with high

percentages (10 percent or more) of truck traffic, the designer should consider providing greater horizontal clearances to vertical sight obstructions to accommodate the greater stopping distances required by large trucks. The approximate HSO required for trucks is

2.5 times the value obtained from Figure 4-A for passenger vehicles.

In designing the roadway to provide a particular stopping sight distance the designer is

advised to consider alternatives. A wider sidewalk, shoulder or bike lane increases the sight triangle, see Section 6-03. Curb extensions and parking restrictions allow the driver

to see pedestrians and cross traffic more easily.

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4.3 Horizontal Alignment

4.3.1 General

In the design of horizontal curves, it is necessary to establish the proper relationship between design speed, curvature and superelevation. Horizontal alignment must afford at

least the minimum stopping sight distance for the design speed at all points on the roadway.

The major considerations in horizontal alignment design are: safety, grade, type of

facility, design speed, topography and construction cost. In design, safety is always considered, either directly or indirectly. Topography controls both curve radius and design

speed to a large extent. The design speed, in turn, controls sight distance, but sight distance must be considered concurrently with topography because it often demands a larger radius than the design speed. All these factors must be balanced to produce an

alignment that is safe, economical, in harmony with the natural contour of the land and, at the same time, adequate for the design classification of the roadway or highway.

4.3.2 Superelevation

When a vehicle travels on a horizontal curve, it is forced radially outward by centrifugal force. This effect becomes more pronounced as the radius of the curve is shortened. This

is counterbalanced by providing roadway superelevation and by the side friction between the vehicle tires and the surfacing. Safe travel at different speeds depends upon the

radius of curvature, the side friction, and the rate of superelevation.

When the standard superelevation for a horizontal curve cannot be met, a design

exception will be required. However, the highest practical superelevation should be selected for the horizontal curve design.

A 6 percent maximum superelevation rate shall be used on rural highways and rural or

urban freeways (see Figure 4-B). A 4 percent maximum superelevation rate may be used on high speed urban highways to minimize conflicts with adjacent development and

intersecting streets (see Figure 4-C). Low speed urban streets can use a 4 percent or 6 percent maximum superelevation rate (see Figure 4-C). The 6 percent maximum superelevation rate for low speed urban streets allows for:

1. a higher threshold of driver discomfort than the 6 percent superelevation rate in Figure 4-B, and

2. application with sharper curvature than the 4 percent maximum superelevation rate in Figure 4-C.

The minimum superelevation to be used is 1.5 percent on flat radius curves requiring

superelevation ranging from 1.5 percent to 2.0 percent, the superelevation should be increased by 0.5 percent in each successive pair of lanes on the low side of the

superelevation when more than two lanes are superelevated in the same direction.

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Values of Superelevation for Rural

Highways and Rural or Urban Freeways

2004 AASHTO

FIGURE 4B

BDC07MR-01

Vd

=

75 m

ph

R(f

t)

15700

11500

10400

9420

8620

7630

7330

6810

6340

5930

5560

5220

4910

4630

4380

4140

3910

3690

3460

3230

2970

2500

Vd

=

70 m

ph

R(f

t)

14100

10300

9240

8380

7660

7030

6490

6010

5580

5210

4860

4550

4270

4010

3770

3550

3330

3120

2910

2700

2460

2040

Vd

=

65 m

ph

R(f

t)

12600

9130

8200

7430

6770

6200

5710

5280

4890

4540

4230

3950

3630

3440

3220

3000

2800

2610

2420

2230

2020

1660

Vd

=

60 m

ph

R(f

t)

11100

8060

7230

6540

5950

5440

4990

4600

4250

3940

3650

3390

3140

2920

2710

2510

2330

2160

1990

1830

1650

1330

Vd

=

55 m

ph

R(f

t)

9410

6820

6110

5520

5020

4580

4200

3860

3560

3290

3040

2810

2590

2400

2210

2050

1890

1750

1610

1470

1320

1060

Vd

=

50 m

ph

R(f

t)

7870

5700

5100

4600

4170

3800

3480

3200

2940

2710

2490

2300

2110

1940

1780

1640

1510

1390

1280

1160

1040

833

Vd

=

45 m

ph

R(f

t)

6480

4680

4190

3770

3420

3110

2840

2600

2390

2190

2010

1840

1680

1540

1410

1300

1190

1090

995

903

806

643

Vd

=

40 m

ph

R(f

t)

5230

3770

3370

3030

2740

2490

2270

2080

1900

1740

1590

1440

1310

1190

1090

995

911

833

759

687

611

485

Vd

=

35 m

ph

R(f

t)

4100

2950

2630

2360

2130

1930

1760

1600

1460

1320

1190

1070

960

868

788

718

654

595

540

487

431

340

Vd

=

30 m

ph

R(f

t)

3130

2240

2000

1790

1610

1460

1320

1200

1080

972

864

766

684

615

555

502

456

413

373

335

296

231

Vd

=

25 m

ph

R(f

t)

2290

1630

1450

1300

1170

1050

944

850

761

673

583

511

452

402

360

324

292

264

237

212

186

144

e

(%)

1.5

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

5.6

5.8

6.0

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Values of Superelevation for

Urban Highways

Note: Use of emax = 4% should be limited to urban conditions.

2004 AASHTO

FIGURE 4C

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e (%)

Vd=

25mph

R (ft)

Vd=

30mph

R (ft)

Vd=

35mph

R (ft)

Vd=

40mph

R (ft)

Vd=

45mph

R (ft)

Vd=

50mph

R (ft)

Vd=

55mph

R (ft)

Vd=

60mph

R (ft) 1.5 2050 2830 3730 4770 5930 7220 8650 10300

2.0 1340 1880 2490 3220 4040 4940 5950 7080

2.2 1110 1580 2120 2760 3480 4280 5180 6190

2.4 838 1270 1760 2340 2980 3690 4500 5410

2.6 650 1000 1420 1930 2490 3130 3870 4700

2.8 524 817 1170 1620 2100 2660 3310 4060

3.0 433 681 983 1370 1800 2290 2860 3530

3.2 363 576 835 1180 1550 1980 2490 3090

3.4 307 490 714 1010 1340 1720 2170 2700

3.6 259 416 610 865 1150 1480 1880 2350

3.8 215 348 512 730 970 1260 1600 2010

4.0 154 250 371 533 711 926 1190 1500

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Notes: 1. Computed using Superelevation Distribution Method 2.

2. Superelevation may be optional on low-speed urban streets.

3. Negative superelevation values beyond -2.0% should be used for low type surfaces such

as gravel, crushed stone, and earth. However, areas with intense rainfall may use normal

cross slopes on high type surfaces of -2.5%.

Values of Superelevation for

Low-Speed Urban Streets

AASHTO 2004

FIGURE 4C1

BDC07MR-01

e (%)

Vd = 25mph

R (ft) Vd = 30mph

R (ft) Vd = 35mph

R (ft) Vd = 40mph

R (ft) Vd = 45mph

R (ft)

-2.6 204 345 530 796 1089

-2.4 202 341 524 784 1071

-2.2 200 337 517 773 1055

-2.0 198 333 510 762 1039

-1.5 194 324 495 736 1000

0 181 300 454 667 900

1.5 170 279 419 610 818

2.0 167 273 408 593 794

2.2 165 270 404 586 785

2.4 164 268 400 580 776

2.6 163 265 396 573 767

2.8 161 263 393 567 758

3.0 160 261 389 561 750

3.2 159 259 385 556 742

3.4 158 256 382 550 734

3.6 157 254 378 544 726

3.8 155 252 375 539 718

4.0 154 250 371 533 711

4.2 153 248 368 528 703

4.4 152 246 365 523 696

4.6 151 244 361 518 689

4.8 150 242 358 513 682

5.0 149 240 355 508 675

5.2 148 238 352 503 668

5.4 147 236 349 498 662

5.6 146 234 346 494 655

5.8 145 233 343 489 649

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It may be appropriate to provide adverse crown on flat radius curves (less than 2 percent superelevation) to avoid water buildup on the low side of the superelevation when there

are more than three lanes draining across the pavement (This design treatment would require a design exception). Another option is to construct a permeable surface course or

a high macotexture surface course since these surfaces appear to have the highest potential for reducing hydroplaning accidents. Also, grooving the pavement perpendicular to the traveled way may be considered as a corrective measure for severe localized

hydroplaning problems.

Figures 4-B and 4-C give the design values for each rate of superelevation to be used for

various design speeds and radii on mainline curves.

A. Axis of Rotation

1. Undivided Highways

For undivided highways, the axis of rotation for superelevation is usually the centerline of the traveled way. However, in special cases where curves are

preceded by long, relatively level tangents, the plane of superelevation may be rotated about the inside edge of the pavement to improve perception of the curve. In flat terrain, drainage pockets caused by superelevation may be avoided by

changing the axis of rotation from the centerline to the inside edge of the pavement.

2. Ramps and Freeway to Freeway Connections

The axis of rotation may be about either edge of pavement or centerline if

multi-lane. Appearance and drainage considerations should always be taken into account in selection of the axis rotation.

3. Divided Highways

a. Freeways

Where the initial median width is 30 feet or less, the axis of rotation should be

at the median centerline.

Where the initial median width is greater than 30 feet and the ultimate median width is 30 feet or less, the axis of rotation should be at the median centerline,

except where the resulting initial median slope would be steeper than 10H:1V. In the latter case, the axis of rotation should be at the ultimate median edges

of pavement.

Where the ultimate median width is greater than 30 feet, the axis of rotation should be at the proposed median edges of pavement.

Where the initial median width is 30 feet or less, the axis of rotation should be at the median centerline.

Where the initial median width is greater than 30 feet and the ultimate median width is 30 feet or less, the axis of rotation should be at the median centerline, except where the resulting initial median slope would be steeper than 10H:1V.

In the latter case, the axis of rotation should be at the ultimate median edges of pavement.

Where the ultimate median width is greater than 30 feet, the axis of rotation should be at the proposed median edges of pavement.

To avoid a sawtooth on bridges with decked medians, the axis of rotation, if

not already on the median centerline, should be shifted to the median centerline.

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b. Other Divided Highways

The axis of rotation should be considered on an individual project basis and the

most appropriate case for the conditions should be selected.

The selection of the axis of rotation should always be considered in conjunction

with the design of the profile and superelevation transition.

B. Superelevation Transition

The superelevation transition consists of the superelevation runoff (length of roadway

needed to accomplish the change in outside-lane cross slope from zero to full superelevation or vice versa) and tangent runout (length of roadway needed to

accomplish the change in outside-lane cross slope from the normal cross slope to zero or vice versa). The definition of and method of deriving superelevation runoff and runout in this manual is the same as described in AASHTO, “A Policy on Geometric

Design of Highways and Streets.”

The superelevation transition should be designed to satisfy the requirements of safety

and comfort and be pleasing in appearance. The minimum length of superelevation runoff and runout should be based on the following formula:

Superelevation RunofF Tangent Runout

Lr = (w)(n)(e)(b)/ Lt = (Lr)(eNC)/e

Where: Lr = minimum length of

superelevation runoff, ft;

= maximum relative gradient, percent (Table 4-2);

n = number of lanes rotated

b = adjustment factor for number of lanes rotated

(Table 4-3)

w = width of one traffic lane, ft. e = design superelevation rate,

percent

Where Lt = minimum length of tangent

runout, ft.

eNC = normal cross slope rate, percent

e = design superelevation rate

Lr = minimum length of superelevation runoff, ft.

Table 4-2 Maximum Relative Gradient

Design

Speed (mph)

25 30 35 40 45 50 55 60 65 70

Maximum

Relative Gradient

0.70 0.66 0.62 0.58 0.54 0.50 0.47 0.45 0.43 0.4

0

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Table 4-3

Adjustment Factor for Number of Lanes Rotated

Number of Lanes Rotated (n) Adjustment Factor (b)

1 1.00

1.5 0.83

2 0.75

2.5 0.70

3 0.67

3.5 0.64

On 3R projects where the existing runoff and runout lengths are shorter than

calculated from the formula, the existing runoff and runout lengths may be maintained.

With respect to the beginning or ending of a curve, the amount of runoff on the tangent should desirably be based on Table 4-4. However, runoff lengths on the tangent ranging from 60 to 90 percent are acceptable.

Table 4-4

Percent of Runoff on Tangent

Design

Speed

Mph

Portion of runoff located prior to the curve

Number of lanes rotated

1.0 1.5 2.0-2.5 3.0-3.5

25-45 0.80 0.85 0.90 0.90

50-80 0.70 0.75 0.80 0.85

After a superelevation transition is designed, profiles of the edges of pavement and

shoulder should be plotted and irregularities removed by introducing smooth curves by the means of a graphic profile. Flat areas which are undesirable from a drainage standpoint should be avoided.

Pronounced and unsightly sags may develop on the low side of the superelevation. These can be corrected by adjusting the grades on the two edges of pavement

throughout the curve.

C. Transition Curves and Superelevation

The use of transition curves on arterial highways designed for 50 mph or greater is

encouraged. Figures 4D through 4H inclusive indicate the desirable treatment on highway curves including the method of distributing superelevation.

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4.3.3 Curvature

A General

The changes in direction along a highway are basically accounted for by simple curves or compound curves. Excessive curvature or poor combinations of curvature generate

accidents, limit capacity, cause economic losses in time and operating costs, and detract from a pleasing appearance. To avoid these poor design practices, the following general controls should be used.

B. Curve Radii for Horizontal Curves

Table 4-5 gives the minimum radius of open highway curves for specific design speeds.

This table is based upon a 6 percent and 4 percent maximum superelevation; it ignores the horizontal stopping sight distance factor.

Table 4-5

Standards for Curve Radius

Design Speed

(mph)

Minimum Radius of

Curve for Rural

Highways and Rural or Urban

Freeways

Based on 6% emax

(ft)

Minimum

Radius of Curve for

Urban

Highways Based on

4% emax

(ft)

Minimum

Radius of Curve for Low

Speed Urban

Highways Based on

6% emax

(ft)

25

30

35 40

45

50 55

60

65 70

144

231

340 485

643

833 1060

1330

1660 2040

154

250

371 533

711

926 1190

1500

---

144

231

340 485

---

--- ---

---

---

Every effort should be made to exceed the minimum values. Minimum radii should be used only when the cost or other adverse effects of realizing a higher standard are

inconsistent with the benefits. Where a longitudinal barrier is provided in the median, the above minimum radii may need to be increased or the adjacent shoulder widened to provide adequate horizontal stopping sight distance.

The suggested minimum radius for a freeway is 3000 feet in rural areas and 1600 feet in urban areas. For a land service highway, the preferred minimum radius is 1600 feet and

1000 feet for design speeds of 60 mph and 50 mph respectively.

Due to the higher center of gravity on large trucks, sharp curves on open highways may contribute to truck overturning. Overturning becomes critical on radii below approximately

700 feet. Where new or reconstructed curves on open highways with radii less than 700

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feet must be provided, the design of these radii shall be based upon a design speed of at least 10 mph greater than the anticipated posted speed.

C. Alignment Consistency

Sudden reductions in standards introduce the element of surprise to the driver and should be avoided. Where physical restrictions on curve radius cannot be overcome and it

becomes necessary to introduce curvature of a lower standard than the design speed for the project, the design speed between successive curves shall change not more than 10

mph. Introduction of a curve for a design speed lower than the design speed of the project shall be avoided at the end of a long tangent or at other locations where high approach speeds may be anticipated.

D. Stopping Sight Distance

Horizontal alignment should afford at least the desirable stopping sight distance for the

design speed at all points of the highway. Where social, environmental or economic impacts do not permit the use of desirable values, lesser stopping sight distances may be used, but shall not be less than the minimum values.

E. Curve Length and Central Angle

The following is applicable for freeways and rural arterial highways. Desirably, the

minimum curve length for central angles less than 5 degrees should be 500 feet long, and the minimum length should be increased 100 feet for each 1 degree decrease in the central angle to avoid the appearance of a kink. For central angles smaller than 30

minutes, no curve is required. In no event shall sight distance or other safety considerations be sacrificed to meet the above requirement.

F. Compound Curves

On compound curves for arterial highways, the curve treatment shown in Figures 4D through 4H should be used. For compound curves at intersections and ramps, the ratio of

the flatter radius to the sharper radius should not exceed 2.0.

G. Reversing Curves

The intervening tangent distance between reverse curves should, as a minimum, be sufficient to accommodate the superelevation transition as specified in Section 4.3.2, “Superelevation.” For design speeds of 50 mph and greater, longer tangent lengths are

desirable. A range of desirable tangent lengths are shown in Table 4-6 for high design speeds.

Table 4-6 Desirable Tangent Length

Between Reversing Curves

Design Speed

(mph)

Desirable

Tangent

(ft)

50 60

70

500-600 600-800

800-1000

H. Broken Back Curves

A broken back curve consists of two curves in the same direction joined by a short tangent. Broken back curves are unsightly and violate driver expectancy. A reasonable

additional expenditure may be warranted to avoid such curvature.

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The intervening tangent distance between broken back curves should, as a minimum, be sufficient to accommodate the superelevation transition as specified in Section 4.3.2. For

design speeds of 50 mph and greater, longer tangent lengths are desirable. Table 4-7 indicates the desirable tangent length between same direction curves. The desirable

tangent distance should be exceeded when both curves are visible for some distance ahead.

Table 4-7

Desirable Tangent Length

Between Same Direction Curves

Design Speed

(mph)

Desirable

Tangent (ft)

50

60

70

1000

1500

2500

I. Alignment at Bridges

Superelevation transitions on bridges almost always result in an unsightly appearance of the bridge and the bridge railing. Therefore, if at all possible, horizontal curves should

begin and end a sufficient distance from the bridge so that no part of the superelevation transition extends onto the bridge. Alignment and safety considerations, however, are paramount and shall not be sacrificed to meet the above criteria.

4.4 Vertical Alignment

4.4.1 General

The profile line is a reference line by which the elevation of the pavement and other features of the highway are established. It is controlled mainly by topography, type of highway, horizontal alignment, safety, sight distance, construction costs, cultural

development, drainage and pleasing appearance. The performance of heavy vehicles on a grade must also be considered.

All portions of the profile line must meet sight distance requirements for the design speed of the road.

In flat terrain, the elevation of the profile line is often controlled by drainage

considerations. In rolling terrain, some undulation in the profile line is often advantageous, both from the standpoint of truck operation and construction economy.

But, this should be done with appearance in mind; for example, a profile on tangent alignment exhibiting a series of humps visible for some distance ahead should be avoided whenever possible. In rolling terrain, however, the profile usually is closely dependent

upon physical controls.

In considering alternative profiles, economic comparisons should be made. For further

details, see AASHTO, “A Policy on Geometric Design of Highways and Streets.”

4.4.2 Position with Respect to Cross Section

The profile line should generally coincide with the axis of rotation for superelevation. The

relation to the cross section should be as follows:

A. Undivided Highways

The profile line should coincide with the highway centerline.

B. Ramps and Freeway to Freeway Connections

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The profile line may be positioned at either edge of pavement, or centerline of ramp if multi-lane.

C. Divided Highways

The profile line may be positioned at either the centerline of the median or at the

median edge of pavement. The former case is appropriate for paved medians 30 feet wide or less. The latter case is appropriate when:

1. The median edges of pavement of the two roadways are at equal elevation.

2. The two roadways are at different elevations.

4.4.3 Separate Grade Lines

Separate or independent profile lines are appropriate in some cases for freeways and divided arterial highways.

They are not normally considered appropriate where medians are less than 30 feet.

Exceptions to this may be minor differences between opposing grade lines in special situations.

In addition, appreciable grade differentials between roadbeds should be avoided in the vicinity of at-grade intersections. For traffic entering from the crossroad, confusion and wrong-way movements could result if the pavement of the far roadway is obscured due to

an excessive differential.

4.4.4 Standards for Grade

The minimum grade rate for freeways and land service highways with a curbed or bermed section is 0.3 percent. On highways with an umbrella section, grades flatter than 0.3

percent may be used where the shoulder width is 8 feet or greater and the shoulder cross slope is 4 percent or greater. For maximum grades for urban and rural land service highways and freeways, see Table 4-8.

Table 4-8

Maximum Grades (%)

Rural Land Service Highways

Type of

Terrain

Design Speed (mph)

30

40

45

50

55

60

65

Level

---

5

5

4

4

3

3

Rolling ---

6

6

5

5

4

4

Mountainous ---

8

7

7

6

6

5

Urban Land Service Highways

Type of

Terrain

Design Speed (mph)

30

40

45

50

55

60

65

Level

8

7

6

6

5

5

---

Rolling

9

8

7

7

6

6

--- Mountainous

11

10

9

9

8

8

---

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Freeways *

Type of

Terrain

Design Speed (mph)

40

45

50

55

60

65

70

Level

---

---

4

4

3

3

3

Rolling ---

---

5

5

4

4

4

Mountainous ---

---

6

6

6

5

5

* Grades 1% steeper than the value shown may be provided in

mountainous terrain or in urban areas with crucial right-of –way controls.

4.4.5 Vertical Curves

Properly designed vertical curves should provide adequate sight distance, safety,

comfortable driving, good drainage, and pleasing appearance. On new alignments or major reconstruction projects on existing highways, the designer should, where practical,

provide the desirable vertical curve lengths. The use of minimum vertical curve lengths should be limited to existing highways and those locations where the desirable values or values greater than the minimum would involve significant social, environmental or

economic impacts.

A parabolic vertical curve is used to provide a smooth transition between different tangent

grades. Figures 4-I and 4-J give the length of crest and sag vertical curves for various design speeds and algebraic differences in grade. The stopping sight distance for these curves are based upon a height of eye of 3.5 feet, and a height of object of 2 feet. The

minimum length of vertical curve may also be obtained by multiplying the K value (Fig. 4-I or 4-J) by the algebraic difference in grade. The vertical lines in Figure 4-I and 4-J are

equivalent to 3 times the design speed. To determine the length of crest vertical curves on highways designed with two-way left-turn lanes (see Section 6.7.1).

Flat vertical curves may develop poor drainage at level sections. Highway drainage must

be given more careful consideration when the design speed exceeds 60 and 65 mph for crest vertical curves and sag vertical curves respectively. The length of sag vertical curves

for riding comfort should desirably be approximately equal to:

L = AV2/46.5.

L = Length of sag vertical curve, ft. A = Algebraic difference in grades, percent.

V = Design speed, mph.

When the difference between the P.V.I. elevation and the vertical curve elevation at the

P.V.I. station (E) is 0.0625 feet (3/4 inch), a vertical curve is not required. The use of a profile angle point is permitted. The maximum algebraic difference in tangent grades (A)

that an angle point is permitted for various design speeds is shown in Table 4-9. This table is based on a length of vertical curve of 3 times the design speed.

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Table 4-9

Use of a Profile Angle Point

Design Speed (mph)

AMAX %

25

30

35 40

45

50 55

60

65 70

.70

.55

.50

.40

.40

.35

.30

.30

.25

.25

All umbrella section low points in cut and fill sections on freeways and Interstate highways

shall be provided with slope protection at each low point in the mainline or ramp vertical geometry as shown in the “Standard Roadway Construction Details.” The purpose of this

treatment is to minimize maintenance requirements in addressing the gradual build up of a berm which may eventually contribute to water ponding on the roadway surface and/or

erosion of the side slope. The following are some recommended low point treatments:

A. Low Point at Edge of Ramp or Outside Edge of Mainline Pavement

Where practical, an "E" inlet should be provided in the outside edge of pavement at the

low point to catch and divert the surface runoff. Provide outlet protection where needed at the pipe outfall.

As an alternative, provide slope protection which shall consist of the following:

1. Fill Section

Slope protection shall consist of a 20 feet long bituminous concrete paved area

between the edge of pavement and the hinge point (PVI) together with a riprap stone flume on the fill slope and a riprap stone apron at the bottom of the slope.

The riprap shall only be provided where the fill slope is steeper than 4H:1V. Where there is an inlet in a swale at the low point, center the riprap stone apron around the inlet. Where guide rail exists at the low point, the 10 foot long paved area shall

be constructed instead of the non-vegetative surface treatment under the guide rail.

2. Cut Section

Slope protection shall consist of a 20 foot long bituminous concrete paved area between the edge of pavement and the toe of slope.

3. Low Point at Median Edge of Mainline Pavement

Provide slope protection which shall consist of a 20 foot long by 5 foot wide strip of

bituminous concrete pavement adjacent to the inside edge of shoulder. If the fill slope is steeper than 4H:1V, provide riprap stone slope protection as described in "Low Point at Edge of Ramp or Outside Edge of Mainline Pavement".

On two-lane roads, extremely long crest vertical curves over one half mile should be avoided, since many drivers refuse to pass on such curves, despite adequate sight

distance. It is sometimes more economical to use four-lane construction, than to obtain passing sight distance by the use of a long vertical curve.

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Vertical curves affect intersection sight distance, therefore, utilizing the distances in Figure 6-A, an eye height of 3.5 feet and an object height of 3.5 feet; check for vertical

sight distance at the intersection.

Broken back vertical curves consist of two vertical curves in the same direction, separated

by a short grade tangent. A profile with such curvature normally should be avoided.

4.4.6 Heavy Grades

Except on level terrain, often it is not economically feasible to design a profile that will

allow uniform operating speeds for all classes of vehicles. Sometimes, a long sustained gradient is unavoidable.

From a truck operation standpoint, a profile with sections of maximum gradient broken by length of flatter grade is preferable to a long sustained grade only slightly below the maximum allowable. It is considered good practice to use the steeper rates at the bottom

of the grade, thus developing slack for lighter gradient at the top or elsewhere on the grade.

4.4.7 Coordination with Horizontal Alignment

A proper balance between curvature and grades should be sought. When possible, vertical curves should be superimposed on horizontal curves. This reduces the number of sight

distance restrictions on the project, makes changes in profile less apparent, particularly in rolling terrain, and results in a pleasing appearance. For safety reasons, the horizontal

curve should lead the vertical curve. On the other hand, where the change in horizontal alignment at a grade summit is slight, it safely may be concealed by making the vertical

curve overlay the horizontal curve.

When vertical and horizontal curves are thus superimposed, the superelevation may cause distortion in the outer pavement edges. Profiles of the pavement edge should be plotted

and smooth curves introduced to remove any irregularities.

A sharp horizontal curve should not be introduced at or near a pronounced summit or

grade sag. This presents a distorted appearance and is particularly hazardous at night.

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4.5 Climbing Lane

A climbing lane, as shown in Figure 4-K, is an auxiliary lane introduced at the beginning of

a sustained positive grade for the diversion of slow traffic.

Generally, climbing lanes will be provided when the following conditions are satisfied.

These conditions could be waived if slower moving truck traffic was the major contributing factor causing a high accident rate and could be corrected by addition of a climbing lane.

A. Two-Lane Highways

The following three conditions should be satisfied to justify a climbing lane:

1. Upgrade traffic flow rate in excess of 200 vehicles per hour.

2. Upgrade truck flow rate in excess of 20 vehicles per hour.

3. One of the following conditions exists:

a. A 10 mph or greater speed reduction is expected for a typical heavy truck.

b. Level of Service E or F exists on the grade.

c. A reduction of two or more levels of service is experienced when moving from

the approach segment of the grade.

A complete explanation and a sample calculation on how to check for these conditions are shown in the section on "Climbing Lanes" contained in Chapter 3,

“Elements of Design", of the AASHTO, “A Policy on Geometric Design of Highways and Streets.”

B. Freeways and Multi-lane Highways

Both of the following conditions should be satisfied to justify a climbing lane:

1. A 10 mph or greater speed reduction is expected for a typical heavy truck.

2. The service volume on an individual grade should not exceed that attained by using the next poorer level of service from that used for the basic design. The one

exception is that the service volume derived from employing Level of Service D should not be exceeded.

If the analysis indicates that a climbing lane is required, an additional check must be made to determine if the number of lanes required on the grade are sufficient even with a climbing lane.

A complete explanation and a sample calculation on how to check for these conditions are shown in the section on "Climbing Lanes" contained in Chapter 3, “Elements of Design", of

the AASHTO, “A Policy on Geometric Design of Highways and Streets.”

The beginning warrant for a truck climbing lane shall be that point where truck operating speed is reduced by 10 mph. To locate this point, use Exhibit 3-59 or Exhibit 3-60 of the

aforementioned AASHTO Policy, depending on the weight/horsepower ratio of the appropriate truck. The beginning of the climbing lane should be preceded by a tapered

section, desirably 300 feet, however, a 150 feet minimum taper may be used.

Desirably, the point of ending of a climbing lane would be to a point beyond the crest, where a typical truck could attain a speed that is about 10 mph below the operating speed

of the highway. This point can be determined from Exhibit 3-60 of the aforementioned AASHTO Policy. If it is not practical to end the climbing lane as per Exhibit 3-60, end the

climbing lane at a point where the truck has proper sight distance to safely merge into the normal lane, or preferably, 200 feet beyond this point. For two lane highways, passing sight distance should be available. For freeways and multi-lane highways, passing sight

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distance need not be considered. For all highways, as a minimum, stopping sight distance shall be available. The ending taper beyond this point shall be according to Figure 4-L.

A distance-speed profile should be developed for the area of a climbing lane. The profile should start at the bottom of the first long downgrade prior to the upgrade being

considered for a climbing lane, speeds through long vertical curves can be approximated by considering 25 percent of the vertical curve length (chord) as part of the grade under question.

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4.6 Through Lane Transition

Design standards of the various features of the transition between roadways of different

widths should be consistent with the design standards of the superior roadway. The transition for a lane drop or lane width reduction should be made on a tangent section

whenever possible and should avoid locations with horizontal and vertical sight distance restrictions. Whenever feasible, the entire transition should be visible to the driver of a vehicle approaching the narrower section.

The design should be such that at-grade intersections within the transition are avoided.

Figure 4-L shows the minimum required taper length based upon the design speed of the

roadway. In all cases, a taper length longer than the minimum should be provided where possible. In general, when a lane is dropped by tapering, the transition should be on the right so that traffic merges to the left.

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4.1 general superseded - government of new jersey...4.3 horizontal alignment 4.3.1 general in the...

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